Reciprocating piston type engines comprising twin cylinders arranged in a V configuration are the main type of internal combustion engines presently used for highway class motorcycles, generally known as V-twin engines. Each of these V-twin engines contains the same common major internal components having a fixed geometric relationship to each other, consisting of a rotating assembly containing a flywheel and at least one crankpin. Additionally, a piston or ring set would be used in a reciprocating assembly. These two types of assemblies are joined together by a connecting rod with a piston pin at one end and a crankpin at the other end. Due to small physical limitations available within the engine compartment of most motorcycle frames, a designer to improve performance is usually constrained in the selection of optimal cylinder bore size and stroke lengths of the connecting rods. Furthermore, the overall engine height is determined by the sum of the resulting crank case deck height and cylinder length, which in turn defines a connecting rod length for a given engine bore/stroke selection. Usually, it is desirable to minimize engine height to gain advantages by providing a lower center of gravity for handling purposes and also to provide a lower seat height to aid in the comfort of the operator, as well as any passengers.
A second design limitation is due to the proper piston selection for a given engine bore/stroke design. At the lowest point of the connecting rod stroke, the piston is at the bottom of the cylinder. It is known that for greater engine reliability, it is advantageous that the piston skirt i.e. the length of the piston head, be as long as possible to allow for greater wearing surface between the exterior of the piston skirt and the interior of the cylinder bore. It is also advantageous for the piston skirt to protrude below the crankcase deck height, providing for greater engine rigidity and less torque loading on the cylinder fasteners. However, the length of the piston skirt should be small enough to prevent contacting the flywheels at the bottom of the stroke without increasing the crankcase deck height or the connecting rod length, which would in turn also compromise the engine height.
One of the most common configurations of an air-cooled V-twin design is to mount two cylinders on the same crankcase, generally with one cylinder in front of the second cylinder. This design has the advantage of a very narrow engine that is not as high as the resulting configuration if the same displacement were achieved with only a single cylinder. Additionally, most V-twin type engines are mounted lengthwise within the motorcycle frame, thereby providing the narrowest engine width possible for the operator straddling the engine compartment.
Since modern highway travel allows the motorcycle to travel at higher speeds regardless of the inclusion of additional weight applied to the motorcycle, for example, luggage as well as a passenger, there is an ongoing quest for greater general performance. This quest has resulted in significant increases to V-twin motorcycle engine displacements with the corresponding growth to either bore, stroke or bulk dimensions. Previously, engine displacements were limited not only by physical space limitations on the motorcycle, but also by the reliability and overheating particularly on air-cooled engines. Recent advances in engineering design methods and materials as well as lubricants have, for the most part, mitigated the issues of reliability on larger displacement engines.
However, these newer larger displacement engines still face physical size and space challenges. To increase displacement, either the cylinder bores must increase in volume, or the stroke lengths of the connecting rods must become larger. In the case of increasing the stroke length to achieve more displacement, the engine must inevitably be taller. If this was not the case, dimensional changes would then conflict with intake port (manifold or throttle body) and exhaust part (pipes or headers) alignment and fit. In the case of a bore volume increase beyond that permitted by a typical cylinder wall thickness, crankcases would occasionally need to be over-bored as well or be cast larger resulting in end material thickness and integrity problems. Repositioning larger holes in the crankcase to accept larger bore sizes would result in possible fastener size and location changes or weakening the crankcase structural integrity.
With respect to a V-twin engine, these decisions could also impact the selection of the cylinder angle with respect to the crankcase. Larger cylinder bores would have to be sufficiently spaced apart from one another and could not always be accommodated by merely over-boring the existing cylinders. When larger cylinders are attempted to be placed on the existing crankcase, the cylinder angle generally must be increased to fit the larger displacement cylinders. This problem is compounded by the fact that larger cylinders generally create larger thermal loads and would thereby require larger cooling fins. The utilization of these larger cooling fins would compound the problem even further by compromising the cylinder angle.
A further result of the employment of larger bores and the resulting increased cylinder angle is the noticeable change in engine exhaust acoustics. Utilizing different cylinder angles would necessitate changing the firing intervals within each piston barrel (crank phasing), thereby producing different engine noises than the motorcycle enthusiast has come to expect.
Attempts to increase displacement on air-cooled V-twin motorcycle engines have been accomplished by lengthening the connecting rod stroke and increasing deck height by adding individual cylinder spacer plates, a single crankcase “wedge-type” spacer block or by shortening piston skirts to the point of very low service life and reliability. Other attempts to increase displacement were to build into the crankcase design a taller crankcase with a cast-in increased deck height. Each of these aforementioned attempts is a compromise to either engine reliability or engine height. In every circumstance, there is also a physical limitation to increasing the bore size without having to increase the cylinder angle or compromising the crankcase integrity.
U.S. Pat. No. 6,357,401 to Moriyama et al., U.S. Pat. No. 6,382,169 to Gausman, U.S. Pat. No. 7,174,874 to Liang et al, U.S. Pat. No. 7,703,423 to Burgess et al. and U.S. Pat. No. 7,444,979 to Dondlinger et al. recite typical V-twin engines having one cylinder angled with respect to the second cylinder. As illustrated in these patents, each of the cylinders is independent of one another as, for example, as shown in FIG. 4 of the Gausman reference.
U.S. Patent Application Publication No. 2009/0205591 to Shand illustrates a twin V engine in which the lower portion of the cylinders contact each other as shown in FIGS. 2 and 6. The lower cylindrical skirts of each of the pistons are formed with cutout portions, preventing the pistons from touching each other when they are at the bottom of their stroke as shown in FIG. 6. However, this reference does not adequately address the problem of the prior art relating to motorcycle performance.